The present invention concerns the field of transporting electricity by means of a superconducting link.
In the prior art, electricity is transported by a superconducting cable having superconducting link sections at a temperature which is held much lower than 90 K to maintain the superconducting state of the cable and to prevent the nitrogen from boiling.
FIGS. 1 to 3 show the usual architecture of equipment for transporting current short distances by means of links cooled by liquid nitrogen. Prior art equipment of the above kind includes a set of liquid nitrogen refrigerators (1) arranged equidistantly along the installation. The same flowrate m0 of liquid nitrogen circulates in the go pipe (2) and in the opposite direction in the return pipe (3). The temperatures in the go and return pipes are 70K and 80K, for example.
In the example shown in FIG. 1 two pipes (2, 3) which are thermally insulated from each other are of length 2La, and the distance between two consecutive refrigerators is therefore 2La. FIGS. 2 and 3 show two prior art variants. The operating characteristics are unchanged in the case shown in FIG. 2 of a link half-section of length La fed by a single refrigerator, the liquid nitrogen flowing in a loop having a developed length 2xc3x97La. In the remainder of the description this variant serves as a reference.
The prior art architectures have the drawback of requiring a multiplicity of refrigerators disposed along the link.
The aim of the invention is to overcome this drawback by proposing a method and a link architecture with sections having a length in the order of 100 km, or even several hundred kilometers, the investment and maintenance costs of which are low. To this end, the invention aims to reduce the number of liquefiers and more precisely to replace the multiplicity of low-capacity refrigerators distributed all along each section with a higher capacity liquefaction center and additional means for maintaining the operating temperature of the superconducting cable.
To this end, the invention firstly provides a method of maintaining a superconducting cryolink at low temperature by means of a flow of heat-conducting fluid such as liquid nitrogen produced by a liquefier feeding one end of the link section, wherein the section is fed by a single liquefier and wherein nitrogen is drawn off from at least one intermediate point of the link section.
The liquefier feeds the upstream end of the section with a flow of liquid nitrogen at high pressure, for example 10 atmospheres, to compensate for head losses along the section. All along the section, small quantities of liquid nitrogen are drawn off and expanded in order to reduce the temperature locally. The evaporated nitrogen can be returned to the liquefier via a large-section pipe or tubular member that is not thermally insulated. The evaporated nitrogen can also be ejected to the atmosphere, possibly after heating.
The above method in accordance with the invention imposes a significant limit on the pressure and/or the cross section of the section and/or the number of liquefiers.
In a first variant nitrogen is drawn off in a section including a go pipe and a return pipe by means of a plurality of draw-off stations including a heat exchanger for cooling both pipes.
In a second variant nitrogen is drawn off at a point upstream of the section and a bath of liquid nitrogen is maintained around a heat exchange wall surrounding the pipes.
In a third variant nitrogen is drawn off at a plurality of points distributed along a section comprising a single pipe.
In a fourth variant the pipes are separated in the upstream part of the section and are grouped together at the downstream end of the section.
The upstream pressure of the heat-conveying fluid is advantageously substantially equal to 10 bars.
The invention also provides equipment for transporting electric power via a superconducting cable comprising a pipe surrounding the superconducting cable for circulating a heat-conveying fluid at very low temperature and a liquefier for feeding said pipe, the equipment including at least one draw-off device for drawing off some of the heat-conveying fluid downstream from the liquefier.
The equipment of the invention preferably includes a plurality of draw-off devices each including a heat exchanger.
Advantageously, the downstream part of the pipe has a draw-off orifice fitted with a valve controlled by means for controlling the level of the heat-conveying fluid drawn off, said orifice communicating with a tubular member surrounding the pipe and having an orifice at its upstream end for ejecting gaseous fluid.